This patent application claims priority based upon United Kingdom patent application No. 0006609.2, filed Mar. 17, 2000.
A process for determining the presence of a slag phase in molten steel.
Steel making is considered a batch process. A unit of steel is melted or made with oxygen in a primary steelmaking vessel. The steel is then transferred to a ladle where it is alloyed and refined. Then the steel is transferred to a distribution vessel called a tundish from which it is distributed to one or more molds for solidification. In each of the batch vessels, a slag is present on the steel, comprised of liquid and solid oxides. The properties of the slag are quite different in each batch operation and it is not desirable to allow the transfer of the slag from one vessel to the next in the production sequence.
Slag from the primary steelmaking vessel should not be carried into the ladle, slag formed in the ladle should not be carried into the tundish, and slag from the tundish should not be carried into the molds. At the same time, it is desirable to maximize the yield of metal during transfer operations. Ideally, all the steel and none of the slag should be transferred from one batch operation to the next. Practically, this is not possible since slag and steel tend to form an emulsion or mixture, particularly near the end of a transfer operation. In that case, either some steel must be left untransferred, or some slag must be transferred to the next operation. An object of the present invention is to provide the operator with a tool that will minimize the duration of two phase flow, and to help him to choose by grade the optimum condition for steel retention and slag transfer.
It is known that the degree of mixing of slag and steel during a transfer operation increases with the rate of steel flow. At high flow rates, a vortex may develop well before the end of the transfer operation. In that case, steel and slag may flow together for some time, causing an unacceptable amount of slag transfer. It is the object of steel transfer operations to maintain flow conditions that prevent the mixing and co-transfer of steel and slag. An object of the present invention is to indicate the onset of a vortex and to cause a change in the transfer operation to dissipate the vortex, either automatically or by informing the operator of a recommended course of action.
In the prevention of slag transfer from one vessel to the next, detection of slag flow is important. In many cases, the detection of slag flow is visual and after the fact. For example, in the tapping of steel from an oxygen steel making vessel, the operator will watch the tap stream and the surface of steel in the ladle for indications of slag flow. A significant amount of slag flow causes the stream to brighten and flare due to the higher emissivity and lower surface tension of slag in relation to steel. Also, the lower density of slag causes it to flow across the surface of the steel in the ladle whereas the steel stream penetrates the surface. These indications cause the operator to stop the transfer operation to prevent the further flow of slag into the ladle, but this is usually after significant slag volume has transferred from the steel making vessel into the ladle. The operators vary in level of skill, experience, and attention to detail, causing the amount of slag carry-over to be quite variable from heat to heat. It is therefore desirable to have an operator independent system that can detect the onset of slag flow during the furnace tapping sequence and cause the modification or end of the tapping sequence to minimize the inflow of slag to the ladle.
In another example, when teeming steel from the ladle to the tundish, the operator may watch the pour box area of the tundish for signs of slag flow, such as a brightening of the slag surface around the pouring tube, or a welling up of slag around the pouring tube. Upon seeing these signs, the operator will cause the end of the teeming operation to prevent further flow of slag. Once again, significant slag flow from ladle to tundish may have occurred by that time.
Several aids have been developed to detect slag flow from the ladle. One is based on the difference in conductivity between slag and steel and a resistance is continuously measured between two contact points within the nozzle. This method cannot detect vortexing which often precedes slag flow. Also this method fails if the steel fails to contact one of the probes. Additionally, this method fails if the slag flow is in the center of the stream, allowing the steel to contact both probes and the slag to go undetected.
U.S. Pat. No. 4,140,300 of Gruner et al teaches a method of slag detection that monitors the radiation intensity of the stream of steel flowing through a discharge tube. A lateral side duct is inserted in the ladle shroud, or discharge tube, through which the steel stream can be observed. A change in radiation intensity signals the onset of slag flow. This method is intrusive and requires side duct modification for each shroud, so it has not found acceptance. Additionally, slag flow through the center of the steel stream would go undetected.
Another method of slag detection relies on an indirect method of conductivity measurement using a magnetic field. An electromagnetic coil is placed around the flowing stream of steel. When slag begins to flow, the field properties are changed by the lower conductivity of the stream. The percentage of high conductivity to low conductivity stream area is set at a predetermined rate; and, when this falls below a given threshold, then an alarm signals the operator to shut the ladle stream. Alternatively, the ladle stream can be caused to automatically shut off when the given threshold value is reached. A disadvantage of this method is that vortical flow is not detected. Slag may form only a small percentage of the area of a vortexing stream, and this may not be enough to trigger the alarm to shut the ladle. A further disadvantage of this method is that the electromagnetic coils must be embedded into the refractory bottom of each ladle. These coils require periodic replacement and are a costly maintenance item. Replacement of a damaged coil is usually done when a ladle is scheduled to be relined with new refractory. Until that time, a ladle may go without slag detection ability for several batches of steel.
Yet another method of slag detection relies on the operator""s ability to detect a difference in the vibration of a ladle shroud as slag flow begins. Steel has about twice the density of slag, and it causes the ladle shroud to vibrate significantly as it flows from the ladle into the tundish. This vibration tends to increase in strength during vortexing and decrease in strength during slag flow. Thus, a skilled operator can place a hand on the ladle shroud manipulator arm and sense the vibration during the latter part of a ladle pouring operation. The vibration will abruptly diminish as slag begins to flow through the shroud, at which time the operator causes the termination of the ladle draining operation. A vortex is more difficult to detect by hand, but a skilled operator may also sometimes detect the onset of vortexing flow and may cause the termination of the ladle draining operation at that point. While this method of slag detection is somewhat effective, it relies greatly on the skill and attentiveness of the operator and is thus inconsistent. Also, the operator does not have the ability to discern the various vibration frequencies associated with operations and activities around the casting machine. Some of these may influence his ability to accurately detect slag. In addition, the operator is influenced by his knowledge of approximate weight of steel left in the ladle. His level of sensitivity in slag detection may be low if he perceives that a significant amount of steel remains, and he may miss the early onset of slag. Conversely, his level of sensitivity may be heightened if he perceives that the ladle is almost empty, and he may terminate the pouring operation leaving a significant amount of steel in the ladle.
Instrumentation of the above method of vibration slag detection has been reported in the technical literature in papers from the BHP and NKK steel companies and in the patent literature. In each reported case, an accelerometer was used as a vibration transducer to continuously measure the vibration of the ladle shroud, the shroud manipulator arm or the tundish. In Japanese patent document 58-13455 (January, 1983), there is disclosed monitored vibration of the tundish during steel teeming. The amplified and filtered signal was monitored for a sudden increase and then drop in the amplitude, which signified the onset of slag flow and caused the steel teeming operation to be terminated. The sensitivity of the instrument was increased by also monitoring the rate of change of vibration amplitude with time. K. Yamamoto (Japan 58-13464, January 1983) teaches monitoring the vibration of the ladle shroud to determine the onset of slag flow and automatically terminate teeming. As is the case with the tundish, the shroud vibration amplitude increases with vortexing and decreases with slag flow. In another patent document (Japan Kokai 60-148652, August 1985), there is taught the use of a microphone to monitor the sound of steel flowing through the shroud. The onset of slag flow is marked by a decrease in the sound amplitude. One skilled in the art will realize that sound is, in fact, vibration and the concept is the same as that described in the previously mentioned prior art. The transducer to measure sound may be a piezoelectric accelerometer, a microphone, a Doppler laser device, or any other means that can quantify vibration intensity as a function of time.
BHP Steel in Australia reported using analog signal conditioning to filter out low frequency noise associated with crane movements and other background vibrations. The signal to noise ratio was also improved with analog signal conditioning. They measured vibration amplitude and output the signal using an analog meter. The onset of slag due to vortexing was noted by a marked increase in vibration amplitude. The signal dropped off dramatically after slag flowed through the nozzle. The system sounded a threshold alarm when vortexing reached a critical level, but the operators actually learned to read the analog output and take action based on the vibration pattern observed just before the alarm. The system was reported to be better than visual slag detection or manual vibration detection. It also required less maintenance than electromagnetic methods and was considerably less expensive. However, several disadvantages were noted. Firstly, the flow control slide gate had to be set to a low flow level near the end of a transfer operation which caused reduced metal level in the tundish. Secondly, the slide gate could not be moved after it was set in detection mode. Finally, in conditions where a vortex did not form prior to slag flow, the operator reaction was too slow in closing the teeming stream and significant slag flow into the tundish could occur.
U.S. Pat. No. 5,042,700 teaches the use of a piezoelectric accelerometer to monitor the vibration of the ladle shroud that is caused by steel flowing through it. It is disclosed that vortexing flow causes an increase in vibration amplitude and that the onset of slag flow causes a decrease in vibration amplitude. The vibration signal is continuously compared to a desired signal and action is taken when the vortexing or slag is indicated by the deviation from the desired signal. Additionally, Ardell teaches that a gradual decrease in vibration amplitude over time without adjustment of any flow control device indicates a blockage of the flow channel, such as might occur with the accretion of aluminum oxide particles within the nozzle. The inventors then address the means to clear such a blockage, such as by a burst of argon through the shroud.
In U.S. Pat. No. 5,633,462, it is disclosed that a background vibration signature should be recorded as a comparative signal against which a real time signal is continuously compared. When the real time signal deviates significantly from the background signal, the teeming is caused to stop either by feedback to the flow control device or by alarming the operator. In fact, real time monitoring of vibration, as taught by the prior art, is a continuous comparison of a new amplitude signal to a previous amplitude signal.
It is an object of this invention to provide an improved process for detecting slag.
In accordance with this invention, there is provided a method, for determining the presence of a slag phase in molten steel being transferred in a teeming operation between originating and receiving metallurgical vessels, of the type wherein vibration transferred to structure involved in the transfer is monitored to detect the presence of a slag phase, or conditions conducive to the presence of such a phase, in steel being transferred, the monitored vibrations being processed to provide data used to assess actual or incipient passage of slag, and means operable to control the rate of and termination of teeming is controlled responsive to the data to terminate teeming; wherein parameters of data generated representing characteristics of the vibration are compared with threshold values to generate at least one signal indicative of actual or incipient passage of slag, and the threshold values are progressively adjusted responsive to data collected by monitoring plural parameters of the teeming operation selected from weight of the originating vessel, weight of the receiving vessel, condition of the means controlling teeming, and temperature of the molten steel.